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Ann. Geophys., 20, 629-637, 2002
www.ann-geophys.net/20/629/2002/
© European Geosciences Union 2002
Monitoring magnetosheath-magnetosphere interconnection topology from the aurora
P. E. Sandholt1 and C. J. Farrugia2
1Department of Physics, University of Oslo, Oslo, Norway
2Space Science Center, University of New Hampshire, Durham, New Hampshire, USA
Correspondence to: P. E. Sandholt
(p.e.sandholt@fys.uio.no)
Abstract. A strong southward
rotation of the IMF (BZ from 5 to -6 nT in ~ 20 s) on 4 January 1995
caused an abrupt reconfiguration of midday aurorae and plasma convection
consisting of the following: (1) the red-line aurora associated with
magnetosheath plasma transfer at the low-latitude magnetopause appeared at the
same time that (2) the green-line aurora from precipitating energetic plasma
sheet particles equatorward of the cusp (near the open-closed field line
boundary) weakened visibly and shifted equatorward, (3) the high-latitude
aurora during the previous northward IMF, which is associated with lobe
reconnection, persisted briefly (3 min) and brightened, before it disappeared
from the field-of-view, (4) the activation of a strong convection bay (DPY
current) at cusp and sub-cusp latitudes when the field turned strongly south,
(5) a distinct wave motion of the plasma sheet outer boundary, as inferred from
the aurora, which correlates closely with Pc 5 magnetic pulsations. Our
interpretation of the dramatic reconfiguration is that reconnection poleward of
the cusp coexisted briefly with reconnection at sub-cusp latitudes. The latter
provided a magnetic field connection which enabled, on the one hand,
magnetosheath particles to enter and cause the red-line cusp aurora, and on the
other hand, allowed for magnetospheric energetic particles to escape and weaken
the outer plasma sheet source of the green-line emission. The coexistence of
the two cusp auroras reflects the time required for one field line topology to
replace another, which, under the prevailing high speed wind ( ~ 650 km/s),
lasts ~ 3–4 min. The motion of open flux tubes propagating from equator to
pole during this transition is traced in the aurora by a poleward moving form.
The waves on the outer boundary of the plasma sheet are most likely due to the
Kelvin-Helmholtz instability. The study illustrates the ability of local
auroral observations to monitor even a global change in magnetospheric magnetic
topology.
Key words. Magnetospheric Physics (auroral
phenomena; magnetopause, cusp, and boundary layers; solar wind-magnethoshere
interactions)
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